Subcutaneous cardiac sensing and stimulation system

ABSTRACT

Cardiac systems and methods using ECG and blood information for arrhythmia detection and discrimination. Detection circuitry is configured to produce an ECG. An implantable blood sensor configured to produce a blood sensor signal is coupled to a processor. The processor is coupled to the detection and energy delivery circuitry, and used to evaluate and treat cardiac rhythms using both the cardiac electrophysiologic and blood sensor signals. The blood sensor is configured for subcutaneous non-intrathoracic placement and provided in or on the housing, on a lead coupled to the housing, and/or separate to the housing and coupled to the processor via hardwire or wireless link. The blood sensor may be configured for optical sensing, using a blood oxygen saturation sensor or pulse oximeter. A cardiac rhythm may be evaluated using the electrocardiogram signal and the blood sensor signal, and tachyarrhythmias may be treated after confirmation using the blood sense signal.

RELATED APPLICATIONS

This application is a continuation of U.S. patent application Ser. No.11/975,040, filed Oct. 17, 2007, now U.S. Pat. No. 8,024,039, which is acontinuation of U.S. patent application Ser. No. 10/817,749, filed onApr. 2, 2004, now U.S. Pat. No. 7,302,294, which claims the benefit ofProvisional Patent Application Ser. No. 60/462,272, filed on Apr. 11,2003, to which priority is claimed pursuant to 35 U.S.C. §120 and 35U.S.C. §119(e), respectively, and which are hereby incorporated hereinby reference in their entireties.

FIELD OF THE INVENTION

The present invention relates generally to implantable cardiacmonitoring and stimulation devices and, more particularly, to cardiacsystems and methods using subcutaneously sensed blood information.

BACKGROUND OF THE INVENTION

The healthy heart produces regular, synchronized contractions. Rhythmiccontractions of the heart are normally controlled by the sinoatrial (SA)node, which is a group of specialized cells located in the upper rightatrium. The SA node is the normal pacemaker of the heart, typicallyinitiating 60-100 heartbeats per minute. When the SA node is pacing theheart normally, the heart is said to be in normal sinus rhythm.

If the heart's electrical activity becomes uncoordinated or irregular,the heart is denoted to be arrhythmic. Cardiac arrhythmia impairscardiac efficiency and may be a potential life-threatening event.Cardiac arrhythmias have a number of etiological sources, includingtissue damage due to myocardial infarction, infection, or degradation ofthe heart's ability to generate or synchronize the electrical impulsesthat coordinate contractions.

Bradycardia occurs when the heart rhythm is too slow. This condition maybe caused, for example, by impaired function of the SA node, denotedsick sinus syndrome, or by delayed propagation or blockage of theelectrical impulse between the atria and ventricles. Bradycardiaproduces a heart rate that is too slow to maintain adequate circulation.

When the heart rate is too rapid, the condition is denoted tachycardia.Tachycardia may have its origin in either the atria or the ventricles.Tachycardias occurring in the atria of the heart, for example, includeatrial fibrillation and atrial flutter. Both conditions arecharacterized by rapid contractions of the atria. Besides beinghemodynamically inefficient, the rapid contractions of the atria mayalso adversely affect the ventricular rate.

Ventricular tachycardia occurs, for example, when electrical activityarises in the ventricular myocardium at a rate more rapid than thenormal sinus rhythm. Ventricular tachycardia may quickly degenerate intoventricular fibrillation. Ventricular fibrillation is a conditiondenoted by extremely rapid, uncoordinated electrical activity within theventricular tissue. The rapid and erratic excitation of the ventriculartissue prevents synchronized contractions and impairs the heart'sability to effectively pump blood to the body, which is a fatalcondition unless the heart is returned to sinus rhythm within a fewminutes.

Implantable cardiac rhythm management systems have been used as aneffective treatment for patients with serious arrhythmias. These systemstypically include one or more leads and circuitry to sense signals fromone or more interior and/or exterior surfaces of the heart. Such systemsalso include circuitry for generating electrical pulses that are appliedto cardiac tissue at one or more interior and/or exterior surfaces ofthe heart. For example, leads extending into the patient's heart areconnected to electrodes that contact the myocardium for sensing theheart's electrical signals and for delivering pulses to the heart inaccordance with various therapies for treating arrhythmias.

Typical Implantable cardioverter/defibrillators (ICDs) include one ormore endocardial leads to which at least one defibrillation electrode isconnected. Such ICDs are capable of delivering high-energy shocks to theheart, interrupting the ventricular tachyarrhythmia or ventricularfibrillation, and allowing the heart to resume normal sinus rhythm. ICDsmay also include pacing functionality.

Although ICDs are very effective at preventing Sudden Cardiac Death(SCD), most people at risk of SCD are not provided with implantabledefibrillators. Primary reasons for this unfortunate reality include thelimited number of physicians qualified to perform transvenouslead/electrode implantation, a limited number of surgical facilitiesadequately equipped to accommodate such cardiac procedures, and alimited number of the at-risk patient population that may safely undergothe required endocardial or epicardial lead/electrode implant procedure.

SUMMARY OF THE INVENTION

The present invention is directed to cardiac monitoring and/orstimulation methods and systems that, in general, provide transthoracicmonitoring, defibrillation therapies, pacing therapies, or a combinationof these capabilities. Embodiments of the present invention includethose directed to subcutaneous cardiac monitoring and/or stimulationmethods and systems that evaluate a cardiac rhythm and/or treat acardiac arrhythmia using both ECG and blood information.

According to one embodiment of the invention, a medical device includesa housing configured for subcutaneous non-intrathoracic placement.Detection circuitry is provided in the housing and configured to producea cardiac electrophysiologic signal. Energy delivery circuitry is alsoprovided in the housing. At least one electrode configured forsubcutaneous non-intrathoracic placement is coupled to the detection andenergy delivery circuitry. An implantable blood sensor configured toproduce a blood sensor signal is also provided with the device, andcoupled to a processor provided in the housing. The processor is alsocoupled to the detection and energy delivery circuitry, and used toevaluate a cardiac rhythm using the cardiac electrophysiologic signaland the blood sensor signal. In one approach, the processor isconfigured to use a blood sensor signal to verify that the cardiacelectrophysiologic signal comprises a cardiac signal, and configured toevaluate a cardiac rhythm using the blood sensor signal and the cardiacelectrophysiologic signal comprising the cardiac signal.

The blood sensor may be configured for subcutaneous non-intrathoracicplacement and provided in or on the housing, on a lead coupled to thehousing, and/or separate from the housing and coupled to the processorvia hardwire or wireless link. The blood sensor may include a sensorconfigured for optical signal sensing, such as a blood oxygen saturationsensor or a pulse oximeter. A suitable pulse oximeter may include twolight-emitting diodes and one photodetector. The photodetector mayinclude circuitry having a detection threshold that is periodicallyadjusted to account for signal variations.

In another configuration, a suitable pulse oximeter may include a firstlight-emitting diode having a peak light-emission wavelength within arange of about 550 nm and about 750 nm, and a second light-emittingdiode having a peak light-emission wavelength within a range of about750 nm and about 1050 nm. A photoplethysmography circuit may be includedas a blood sensor and coupled to the processor. The processor mayidentify a cardiac rhythm as a tachyarrhythmia using the cardiacelectrophysiologic signal and the blood sensor signal.

The processor may identify the cardiac rhythm as a tachyarrhythmia usingthe cardiac electrophysiologic signal and a relative change in the bloodsensor signal, and may also selectively activate and deactivate theblood sensor in response to detecting a tachyarrhythmia. The processormay use the cardiac electrophysiologic signal to activate the bloodsensor and evaluate the tachyarrhythmia using the cardiacelectrophysiologic signal and the blood sensor signal. The processor mayfurther confirm or refute the presence of the tachyarrhythmia using thecardiac electrophysiologic signal and the blood sensor signal.

The device may deliver a therapy to treat a tachyarrhythmia, and theprocessor may deactivate the blood sensor before or after delivery ofthe therapy. The processor may determine a hemodynamic state using thecardiac electrophysiologic signal and the blood sensor signal. Inresponse to detecting an unidentifiable cardiac rhythm using the cardiacelectrophysiologic signal, the processor may activate the blood sensorto facilitate identification of the unidentifiable cardiac rhythm usingthe blood sensor signal. The processor may use the blood sensor signalto assess cardiac function, assess oxygen saturation and changes inoxygen saturation, and/or assess afterload by, for example, analyzingthe morphology of the blood sensor signal.

Embodiments of rhythm evaluation methods in accordance with the presentinvention may involve sensing an electrocardiogram signal at asubcutaneous non-intrathoracic location and acquiring a blood sensesignal from a subcutaneous non-intrathoracic sensing location. A cardiacrhythm may be evaluated using the electrocardiogram signal and the bloodsensor signal. One approach involves verifying that theelectrocardiogram signal comprises a cardiac signal, and evaluating acardiac rhythm using the blood sense signal and the electrocardiogramsignal comprising the cardiac signal. A tachyarrhythmia may be detectedusing one or both of the electrocardiogram signal and the blood sensesignal, such as by performing a rate based analysis or by performing amorphology based analysis.

An activation pattern of the electrocardiogram signal may be analyzedusing a plurality of electrodes, and detected tachyarrhythmias may betreated after confirming presence of the tachyarrhythmia using the bloodsense signal. The tachyarrhythmia may be discerned from noise using theblood sense signal. Evaluating the cardiac rhythm may also involvedetecting a cardiac arrhythmia by performing a correlation (or computinga transfer function) between the electrocardiogram signal and the bloodsense signal. Acquiring the blood sense signal may involve selectivelypowering-up and powering-down a blood sensor that produces the bloodsense signal.

Evaluating a cardiac rhythm may involve detecting a tachyarrhythmiausing the electrocardiogram signal, powering-up a blood sensor thatproduces the blood sense signal, confirming presence of thetachyarrhythmia using the blood sense signal, and then powering-down theblood sensor. The blood sense signal may include, for example, bloodperfusion information, blood oxygen saturation information,photoplethysmographic information, pulse oximetry information, and/orother information from a blood sensor.

The above summary of the present invention is not intended to describeeach embodiment or every implementation of the present invention.Advantages and attainments, together with a more complete understandingof the invention, will become apparent and appreciated by referring tothe following detailed description and claims taken in conjunction withthe accompanying drawings.

BRIEF DESCRIPTION OF THE DRAWINGS

FIGS. 1A and 1B are views of a transthoracic cardiac sensing and/orstimulation device as implanted in a patient in accordance with anembodiment of the present invention;

FIG. 1C is a block diagram showing various components of a transthoraciccardiac sensing and/or stimulation device in accordance with anembodiment of the present invention;

FIG. 1D is a block diagram illustrating various processing and detectioncomponents of a transthoracic cardiac sensing and/or stimulation devicein accordance with an embodiment of the present invention;

FIG. 1E is a block diagram illustrating one configuration of variousITCS device components in accordance with an embodiment of the presentinvention;

FIG. 2 is a plan view of a subcutaneously implanted ICD withphotoplethysmography capability in accordance with an embodiment of thepresent invention;

FIG. 3 is a block diagram illustrating a two-color photoplethysmographicsystem in accordance with an embodiment of the present invention;

FIG. 4 is a graph illustrating signals from normal sinus rhythm versusventricular fibrillation;

FIG. 5 is a graph illustrating RMS photoplethysmogram levels duringnormal sinus rhythm versus ventricular fibrillation;

FIGS. 6A and 6B are circuit diagrams of an LED transmission circuit andan LED detection circuit in accordance with an embodiment of the presentinvention; and

FIG. 7 is a flow chart of a method of arrhythmia discrimination inaccordance with an embodiment of the present invention.

While the invention is amenable to various modifications and alternativeforms, specifics thereof have been shown by way of example in thedrawings and will be described in detail below. It is to be understood,however, that the intention is not to limit the invention to theparticular embodiments described. On the contrary, the invention isintended to cover all modifications, equivalents, and alternativesfalling within the scope of the invention as defined by the appendedclaims.

DETAILED DESCRIPTION OF VARIOUS EMBODIMENTS

In the following description of the illustrated embodiments, referencesare made to the accompanying drawings that form a part hereof, and inwhich is shown by way of illustration, various embodiments in which theinvention may be practiced. It is to be understood that otherembodiments may be utilized, and structural and functional changes maybe made without departing from the scope of the present invention.

System and methods of the present invention may include one or more ofthe features, structures, methods, or combinations thereof describedhereinbelow. For example, implantable cardiac systems and methods inaccordance with the present invention may be implemented to include oneor more of the advantageous features and/or processes described below.It is intended that such devices and methods need not include all of thefeatures described herein, but may be implemented to include selectedfeatures that provide for unique structures and/or functionality. Suchdevices and methods may be implemented to provide a variety oftherapeutic and/or diagnostic functions.

Embodiments of the present invention are directed to implantable cardiacdevices (ICDs) having incorporated blood sensor capabilities.Embodiments of the present invention are also directed to systems andmethods for discriminating between arrhythmia and normal sinus rhythm(NSR) using blood sensor information. One such implantable device,termed an implantable transthoracic cardiac sensing and/or stimulation(ITCS) device, is described herein to include various advantageousfeatures and/or processes. It is understood that the description offeatures and processes within the context of an ITCS device is providedfor non-limiting illustrative purposes only. For example, variousfeatures and processes described herein may be implemented for devicessuch as cardiac monitors, diagnostic devices, pacemakers,cardioverters/defibrillators, resynchronizers, and the like, includingthose devices disclosed in the various patents incorporated herein byreference.

In general terms, the ITCS device may be implanted under the skin in thechest region of a patient. The ITCS device may, for example, beimplanted subcutaneously such that all or selected elements of thedevice are located on the patient's front, back, side, or other bodylocations suitable for sensing cardiac activity and delivering cardiacstimulation therapy. It is understood that elements of the ITCS devicemay be located at several different body locations, such as in thechest, abdominal, or subclavian region with electrode elementsrespectively located at different regions near, around, in, or on theheart. Examples of electrode configurations, elements of which may belocated in accordance with the present invention, are disclosed incommonly owned U.S. patent application Ser. No. 10/465,520, filed Jun.19, 2003, which is hereby incorporated herein by reference in itsentirety.

In one configuration, the primary housing (e.g., the active ornon-active can) of the ITCS device, for example, may be configured forpositioning outside of the rib cage at an intercostal or subcostallocation, within the abdomen, or in the upper chest region (e.g.,subclavian location, such as above the third rib). In oneimplementation, one or more electrodes may be located on the primaryhousing and/or at other locations about, but not in direct contact withthe heart, great vessel or coronary vasculature. In anotherimplementation, one or more electrodes may be located in direct contactwith the heart, great vessel or coronary vasculature, such as via one ormore leads. In another implementation, for example, one or moresubcutaneous electrode subsystems or electrode arrays may be used tosense cardiac activity and deliver cardiac stimulation energy in an ITCSdevice configuration employing an active can or a configurationemploying a non-active can. Electrodes may be situated at anteriorand/or posterior locations relative to the heart.

Certain configurations illustrated herein are generally described ascapable of implementing various functions traditionally performed by animplantable cardioverter/defibrillator (ICD), and may operate innumerous cardioversion/defibrillation modes as are known in the art.Examples of ICD circuitry, structures and functionality, elements ofwhich may be incorporated in an ITCS device of a type contemplatedherein, are disclosed in commonly owned U.S. Pat. Nos. 5,133,353;5,179,945; 5,314,459; 5,318,597; 5,620,466; and 5,662,688, which arehereby incorporated herein by reference in their respective entireties.

In particular configurations, systems and methods may perform functionstraditionally performed by pacemakers, such as providing various pacingtherapies as are known in the art, in addition tocardioversion/defibrillation therapies. Examples of pacemaker circuitry,structures and functionality, elements of which may be incorporated inan ITCS device of a type contemplated herein, are disclosed in commonlyowned U.S. Pat. Nos. 4,562,841; 5,284,136; 5,036,849; 5,376,106;5,540,727; 5,836,987; 6,044,298; and 6,055,454, which are herebyincorporated herein by reference in their respective entireties. It isunderstood that ITCS device configurations may provide fornon-physiologic pacing support in addition to, or to the exclusion of,bradycardia and/or anti-tachycardia pacing therapies.

An ITCS device may implement functionality traditionally provided bycardiac diagnostic devices or cardiac monitors as are known in the art,alternatively or additionally to providing cardioversion/defibrillationtherapies. Examples of cardiac monitoring circuitry, structures andfunctionality, elements of which may be incorporated in an ITCS deviceof a type contemplated herein, are disclosed in commonly owned U.S. Pat.Nos. 5,313,953; 5,388,578; and 5,411,031, which are hereby incorporatedherein by reference in their respective entireties.

An ITCS device may implement various anti-tachyarrhythmia therapies,such as tiered therapies. Subcutaneous, cutaneous, and/or external bloodsensors may be employed to acquire information for purposes of enhancingtachyarrhythmia detection and termination. It is understood thatconfigurations, features, and combination of features described in theinstant disclosure may be implemented in a wide range of implantablemedical devices, and that such embodiments and features are not limitedto the particular devices described herein.

An ITCS device may be used to implement various diagnostic functions,which may involve performing rate-based, pattern and rate-based, and/ormorphological tachyarrhythmia discrimination analyses. Subcutaneous,cutaneous, and/or external blood sensors may be employed to acquireinformation for purposes of enhancing tachyarrhythmia detection andtermination. It is understood that configurations, features, andcombination of features described in the instant disclosure may beimplemented in a wide range of implantable medical devices, and thatsuch embodiments and features are not limited to the particular devicesdescribed herein.

Referring now to FIGS. 1A and 1B of the drawings, there is shown aconfiguration of an ITCS device implanted in the chest region of apatient at different locations. In the particular configuration shown inFIGS. 1A and 1B, the ITCS device includes a housing 102 within whichvarious cardiac sensing, detection, processing, and energy deliverycircuitry may be housed. It is understood that the components andfunctionality depicted in the figures and described herein may beimplemented in hardware, software, or a combination of hardware andsoftware. It is further understood that the components and functionalitydepicted as separate or discrete blocks/elements in the figures may beimplemented in combination with other components and functionality, andthat the depiction of such components and functionality in individual orintegral form is for purposes of clarity of explanation, and not oflimitation.

Communications circuitry is disposed within the housing 102 forfacilitating communication between the ITCS device and an externalcommunication device, such as a portable or bed-side communicationstation, patient-carried/worn communication station, or externalprogrammer, for example. The communications circuitry may alsofacilitate unidirectional or bidirectional communication with one ormore external, cutaneous, or subcutaneous physiologic or non-physiologicsensors such as blood sensors used in accordance with embodiments of thepresent invention. The housing 102 is typically configured to includeone or more electrodes (e.g., can electrode and/or indifferentelectrode). Although the housing 102 is typically configured as anactive can, it is appreciated that a non-active can configuration may beimplemented, in which case at least two electrodes spaced apart from thehousing 102 are employed.

In the configuration shown in FIGS. 1A and 1B, a subcutaneous electrode104 may be positioned under the skin in the chest region and situateddistal from the housing 102. The subcutaneous and, if applicable,housing electrode(s) may be located about the heart at various locationsand orientations, such as at various anterior and/or posterior locationsrelative to the heart. The subcutaneous electrode 104 is electricallycoupled to circuitry within the housing 102 via a lead assembly 106. Oneor more conductors (e.g., coils or cables) are provided within the leadassembly 106 and electrically couple the subcutaneous electrode 104 withcircuitry in the housing 102. One or more sense, sense/pace ordefibrillation electrodes may be situated on the elongated structure ofthe electrode support, the housing 102, and/or the distal electrodeassembly (shown as subcutaneous electrode 104 in the configuration shownin FIGS. 1A and 1B).

In accordance with a further configuration, the lead assembly 106includes a rigid electrode support assembly, such as a rigid elongatedstructure that positionally stabilizes the subcutaneous electrode 104with respect to the housing 102. In this configuration, the rigidity ofthe elongated structure maintains a desired spacing between thesubcutaneous electrode 104 and the housing 102, and a desiredorientation of the subcutaneous electrode 104/housing 102 relative tothe patient's heart. The elongated structure may be formed from astructural plastic, composite or metallic material, and includes, or iscovered by, a biocompatible material. Appropriate electrical isolationbetween the housing 102 and subcutaneous electrode 104 is provided incases where the elongated structure is formed from an electricallyconductive material, such as metal.

In one configuration, the rigid electrode support assembly and thehousing 102 define a unitary structure (i.e., a single housing/unit).The electronic components and electrode conductors/connectors aredisposed within or on the unitary ITCS device housing/electrode supportassembly. At least two electrodes are supported on the unitary structurenear opposing ends of the housing/electrode support assembly. Theunitary structure may have an arcuate or angled shape, for example.

According to another configuration, the rigid electrode support assemblydefines a physically separable unit relative to the housing 102. Therigid electrode support assembly includes mechanical and electricalcouplings that facilitate mating engagement with correspondingmechanical and electrical couplings of the housing 102. For example, aheader block arrangement may be configured to include both electricaland mechanical couplings that provide for mechanical and electricalconnections between the rigid electrode support assembly and housing102. The header block arrangement may be provided on the housing 102 orthe rigid electrode support assembly. Alternatively, amechanical/electrical coupler may be used to establish mechanical andelectrical connections between the rigid electrode support assembly andhousing 102. In such a configuration, a variety of different electrodesupport assemblies of varying shapes, sizes, and electrodeconfigurations may be made available for physically and electricallyconnecting to a standard ITCS device housing 102.

An ITCS device may incorporate circuitry, structures and functionalityof the subcutaneous implantable medical devices disclosed in commonlyowned U.S. Pat. Nos. 5,203,348; 5,230,337; 5,360,442; 5,366,496;5,391,200; 5,397,342; 5,545,202; 5,603,732; and 5,916,243, which arehereby incorporated herein by reference in their respective entireties.

FIG. 1C is a block diagram depicting various components of an ITCSdevice in accordance with one configuration. According to thisconfiguration, the ITCS device incorporates a processor-based controlsystem 205 which includes a micro-processor 206 coupled to appropriatememory (volatile and/or non-volatile) 209, it being understood that anylogic-based control architecture may be used. The control system 205 iscoupled to circuitry and components to sense, detect, and analyzeelectrical signals produced by the heart and deliver electricalstimulation energy to the heart under predetermined conditions to treatcardiac arrhythmias. In certain configurations, the control system 205and associated components also provide pacing therapy to the heart. Theelectrical energy delivered by the ITCS device may be in the form of lowenergy pacing pulses or high-energy pulses for cardioversion ordefibrillation.

Cardiac signals are sensed using the subcutaneous electrode(s) 214 andthe can or indifferent electrode 207 provided on the ITCS devicehousing. Cardiac signals may also be sensed using only the subcutaneouselectrodes 214, such as in a non-active can configuration. As such,unipolar, bipolar, or combined unipolar/bipolar electrode configurationsmay be employed. The sensed cardiac signals are received by sensingcircuitry 204, which includes sense amplification circuitry and may alsoinclude filtering circuitry and an analog-to-digital (A/D) converter.The sensed cardiac signals processed by the sensing circuitry 204 may bereceived by noise reduction circuitry 203, which may further reducenoise for signals used by the detection circuitry 202. Noise reductioncircuitry 203 may also be incorporated after detection circuitry 202 incases where high power or computationally intensive noise reductionalgorithms are required.

In the illustrative configuration shown in FIG. 1C, the detectioncircuitry 202 is coupled to, or otherwise incorporates, noise reductioncircuitry 203. The noise reduction circuitry 203 operates to improve thesignal-to-noise ratio of sensed cardiac signals by removing noisecontent of the sensed cardiac signals introduced from various sources.Typical types of transthoracic cardiac signal noise includes electricalnoise and noise produced from skeletal muscles, for example.

Detection circuitry 202 typically includes a signal processor thatcoordinates analysis of the sensed cardiac signals and/or other sensorinputs to detect cardiac arrhythmias, such as, in particular,tachyarrhythmia. Rate based and/or morphological discriminationalgorithms may be implemented by the signal processor of the detectioncircuitry 202 to detect and verify the presence and severity of anarrhythmic episode. Examples of arrhythmia detection and discriminationcircuitry, structures, and techniques, elements of which may beimplemented by an ITCS device of a type contemplated herein, aredisclosed in commonly owned U.S. Pat. Nos. 5,301,677 and 6,438,410,which are hereby incorporated herein by reference in their respectiveentireties.

The detection circuitry 202 communicates cardiac signal information tothe control system 205. Memory circuitry 209 of the control system 205contains parameters for operating in various sensing, defibrillation,and pacing modes, and stores data indicative of cardiac signals receivedby the detection circuitry 202. The memory circuitry 209 may also beconfigured to store historical ECG, blood information, and therapy data,which may be used for various purposes and transmitted to an externalreceiving device as needed or desired.

In certain configurations, the ITCS device may include diagnosticscircuitry 210. The diagnostics circuitry 210 typically receives inputsignals from the detection circuitry 202 and the sensing circuitry 204.The diagnostics circuitry 210 provides diagnostics data to the controlsystem 205, it being understood that the control system 205 mayincorporate all or part of the diagnostics circuitry 210 or itsfunctionality. The control system 205 may store and use informationprovided by the diagnostics circuitry 210 for a variety of diagnosticspurposes. This diagnostic information may be stored, for example,subsequent to a triggering event or at predetermined intervals, and mayinclude system diagnostics, such as power source status, therapydelivery history, and/or patient diagnostics. The diagnostic informationmay take the form of electrical signals or other sensor data acquiredimmediately prior to therapy delivery.

According to a configuration that provides cardioversion anddefibrillation therapies, the control system 205 processes cardiacsignal data received from the detection circuitry 202 and initiatesappropriate tachyarrhythmia therapies to terminate cardiac arrhythmicepisodes and return the heart to normal sinus rhythm. The control system205 is coupled to shock therapy circuitry 216. The shock therapycircuitry 216 is coupled to the subcutaneous electrode(s) 214 and thecan or indifferent electrode 207 of the ITCS device housing. Uponcommand, the shock therapy circuitry 216 delivers cardioversion anddefibrillation stimulation energy to the heart in accordance with aselected cardioversion or defibrillation therapy. In a lesssophisticated configuration, the shock therapy circuitry 216 iscontrolled to deliver defibrillation therapies, in contrast to aconfiguration that provides for delivery of both cardioversion anddefibrillation therapies. Examples of ICD high energy deliverycircuitry, structures and functionality, elements of which may beincorporated in an ITCS device of a type contemplated herein, aredisclosed in commonly owned U.S. Pat. Nos. 5,372,606; 5,411,525;5,468,254; and 5,634,938, which are hereby incorporated herein byreference in their respective entireties.

In accordance with another configuration, an ITCS device may incorporatea cardiac pacing capability in addition to cardioversion and/ordefibrillation capabilities. As is shown in dotted lines in FIG. 1C, theITCS device may include pacing therapy circuitry 230, which is coupledto the control system 205 and the subcutaneous and can/indifferentelectrodes 214, 207. Upon command, the pacing therapy circuitry deliverspacing pulses to the heart in accordance with a selected pacing therapy.Control signals, developed in accordance with a pacing regimen bypacemaker circuitry within the control system 205, are initiated andtransmitted to the pacing therapy circuitry 230 where pacing pulses aregenerated. A pacing regimen may be modified by the control system 205.

A number of cardiac pacing therapies are particularly useful in atransthoracic cardiac stimulation device. Such cardiac pacing therapiesmay be delivered via the pacing therapy circuitry 230 as shown in FIG.1C. Alternatively, cardiac pacing therapies may be delivered via theshock therapy circuitry 216, which effectively obviates the need forseparate pacemaker circuitry.

The ITCS device shown in FIG. 1C may be configured to receive signalsfrom one or more physiologic and/or non-physiologic sensors 212.Depending on the type of sensor employed, signals generated by thesensors may be communicated to transducer circuitry coupled directly tothe detection circuitry or indirectly via the sensing circuitry. It isnoted that certain sensors may transmit sense data to the control system205 without processing by the detection circuitry 202.

Communications circuitry 218 is coupled to the microprocessor 206 of thecontrol system 205. The communications circuitry 218 allows the ITCSdevice to communicate with one or more receiving devices or systemssituated external to the ITCS device. By way of example, the ITCS devicemay communicate with a patient-worn, portable or bedside communicationsystem via the communications circuitry 218. In one configuration, oneor more physiologic or non-physiologic sensors (subcutaneous, cutaneous,or external of patient) may be equipped with a short-range wirelesscommunication interface, such as an interface conforming to a knowncommunications standard, such as Bluetooth or IEEE 802 standards. Dataacquired by such sensors may be communicated to the ITCS device via thecommunications circuitry 218. It is noted that physiologic ornon-physiologic sensors equipped with wireless transmitters ortransceivers may communicate with a receiving system external of thepatient.

The communications circuitry 218 may allow the ITCS device tocommunicate with an external programmer. In one configuration, thecommunications circuitry 218 and the programmer unit (not shown) use awire loop antenna and a radio frequency telemetric link, as is known inthe art, to receive and transmit signals and data between the programmerunit and communications circuitry 218. In this manner, programmingcommands and data are transferred between the ITCS device and theprogrammer unit during and after implant. Using a programmer, aphysician is able to set or modify various parameters used by the ITCSdevice. For example, a physician may set or modify parameters affectingsensing, detection, pacing, and defibrillation functions of the ITCSdevice, including pacing and cardioversion/defibrillation therapy modes.

Typically, the ITCS device is encased and hermetically sealed in ahousing suitable for implanting in a human body as is known in the art.Power to the ITCS device is supplied by an electrochemical power source220 housed within the ITCS device. In one configuration, the powersource 220 includes a rechargeable battery. According to thisconfiguration, charging circuitry is coupled to the power source 220 tofacilitate repeated non-invasive charging of the power source 220. Thecommunications circuitry 218, or separate receiver circuitry, isconfigured to receive RF energy transmitted by an external RF energytransmitter. The ITCS device may, in addition to a rechargeable powersource, include a non-rechargeable battery. It is understood that arechargeable power source need not be used, in which case a long-lifenon-rechargeable battery is employed.

FIG. 1D illustrates a configuration of detection circuitry 302, which iscoupled to a micro-processor 306, of an ITCS device that includes one orboth of rate detection circuitry 310 and morphological analysiscircuitry 312 in combination with blood sensor processing circuitry 318.Detection and verification of arrhythmias may be accomplished usingrate-based discrimination algorithms implemented by the rate detectioncircuitry 310 in combination with blood sensor processing circuitry 318.Arrhythmic episodes may also be detected and verified bymorphology-based analysis implemented by the morphology analysiscircuitry 312 in combination with blood sensor processing circuitry 318.Tiered or parallel arrhythmia discrimination algorithms may also beimplemented using both rate-based and morphologic-based approaches.Further, a rate and pattern-based arrhythmia detection anddiscrimination approach may be employed to detect and/or verifyarrhythmic episodes, such as the approaches disclosed in U.S. Pat. Nos.6,487,443; 6,259,947; 6,141,581; 5,855,593; and 5,545,186, which arehereby incorporated herein by reference in their respective entireties.

An ITCS device of a type described herein may be used within thestructure of an advanced patient management (APM) system. Advancedpatient management systems may allow physicians to remotely andautomatically monitor cardiac and respiratory functions, as well asother patient conditions. In one example, implantable cardiac rhythmmanagement systems, such as cardiac pacemakers, defibrillators, andresynchronization devices, may be equipped with varioustelecommunications and information technologies such as cardiac signalnoise processing circuitry 314 and skeletal muscle signal processingcircuitry 316 that enable real-time data collection, diagnosis, andtreatment of the patient. Various embodiments described herein may beused in connection with advanced patient management. Methods,structures, and/or techniques described herein, which may be adapted toprovide for remote patient/device monitoring, diagnosis, therapy, orother APM related methodologies, may incorporate features of one or moreof the following references: U.S. Pat. Nos. 6,221,011; 6,270,457;6,277,072; 6,280,380; 6,312,378; 6,336,903; 6,358,203; 6,368,284;6,398,728; and 6,440,066, which are hereby incorporated herein byreference.

An ITCS device of a type described herein may be used within thestructure of an advanced patient management (APM) system. Advancedpatient management systems may allow physicians to remotely andautomatically monitor cardiac and respiratory functions, as well asother patient conditions. In one example, implantable cardiac rhythmmanagement systems, such as cardiac pacemakers, defibrillators, andresynchronization devices, may be equipped with varioustelecommunications and information technologies that enable real-timedata collection, diagnosis, and treatment of the patient. Variousembodiments described herein may be used in connection with advancedpatient management. Methods, structures, and/or techniques describedherein, which may be adapted to provide for remote patient/devicemonitoring, diagnosis, therapy, or other APM related methodologies, mayincorporate features of one or more of the following references: U.S.Pat. Nos. 6,221,011; 6,270,457; 6,277,072; 6,280,380; 6,312,378;6,336,903; 6,358,203; 6,368,284; 6,398,728; and 6,440,066, which arehereby incorporated herein by reference.

Turning now to FIG. 1E, there is illustrated a block diagram of variouscomponents of an ITCS device in accordance with one configuration. FIG.1E illustrates a number of components that are associated with detectionof various physiologic and non-physiologic parameters. As shown, theITCS device includes a micro-processor 406, which is typicallyincorporated in a control system for the ITCS device, coupled todetection circuitry 402, patient activator 432, and optionally, cardiacdrug delivery device 430. Sensor signal processing circuitry 410 canreceive sensor data from a number of different sensors.

For example, an ITCS device may cooperate with, or otherwiseincorporate, various types of non-physiologic sensors 421,external/cutaneous physiologic sensors 422, and/or internal physiologicsensors 424. Such sensors may include an acoustic sensor, an impedancesensor, an oxygen saturation sensor, a blood volume sensor, and a bloodpressure sensor, for example. Each of these sensors 421, 422, 424 may becommunicatively coupled to the sensor signal processing circuitry 410via a short range wireless communication link 420. Certain sensors, suchas an internal physiologic sensor 424, may alternatively becommunicatively coupled to the sensor signal processing circuitry 410via a wired connection (e.g., electrical or optical connection). Auseful photoplethysmography sensor and techniques for using same thatmay be implemented in an ITCS device of the present invention aredisclosed in U.S. Pat. No. 6,491,639, which is hereby incorporatedherein by reference.

The components, functionality, and structural configurations depicted inFIGS. 1A-1E are intended to provide an understanding of various featuresand combination of features that may be incorporated in an ITCS device.It is understood that a wide variety of ITCS and other implantablecardiac monitoring and/or stimulation device configurations arecontemplated, ranging from relatively sophisticated to relatively simpledesigns. As such, particular ITCS or cardiac monitoring and/orstimulation device configurations may include particular features asdescribed herein, while other such device configurations may excludeparticular features described herein.

Electrocardiogram signals often contain noise signals and artifacts thatmimic true cardiac signals and various arrhythmias. Using a blood sensorin accordance with the present invention provides the ability todiscriminate true arrhythmia conditions from various noisy conditions.Moreover, using a blood sensor in accordance with the present inventionprovides the ability to confirm that the signals upon which arrhythmiadetection and therapy delivery decisions are made contain a cardiacsignal (e.g., a QRS complex), rather than a spurious signal that mayhave features similar to those of a true cardiac signal. An ITCS devicemay be implemented to include multi-parameter cardiac signalverification and/or arrhythmia discrimination capabilities to improvenoise rejection of cardiac ECG signals sensed by subcutaneouselectrodes. This noise rejection/reduction approach advantageouslyreduces the risk of false positives for detection algorithms byproviding multi-parameter arrhythmia discrimination.

For example, a non-electrophysiologic signal may be used to verify thatthe ECG signal contains a cardiac signal having a QRS complex, and thatonly ECG signals with QRS complexes are considered verified ECG signals.Subsequent cardiac rhythm analyses, including, in particular, arrhythmiaanalyses, may require that only verified ECG signals are used forcomputations of, for example, heart rate used for such analyses. Thiscardiac signal confirmation technique provides for more robustalgorithms that are less susceptible to contamination from electricalinterference and noise, thereby reducing incidences of inappropriatetachyarrhythmia therapy delivery.

One approach to cardiac signal confirmation involves determiningtemporal relationships between electrocardiogram signals andnon-electrophysiologic signals. A detection window may, for example, beinitiated in response to detecting an electrocardiogram signal, and usedto determine whether a non-electrophysiologic signal is or is notreceived at a time falling within the detection window. For example, onearrhythmia detection approach uses the ECG signal to define a detectionwindow. A non-electrophysiological source signal, such as a blood sensorsignal, is then evaluated within the detection window for cardiacinformation. If the non-electrophysiological source signal includes acardiac event within the window, then the ECG signal is corroborated ascorresponding to a cardiac event. This may be used, for example, in arate-based arrhythmia detection algorithm to provide a more robust ratethan the rate calculated if only ECG information is used. The algorithmmay, for example, only count ECG identified heart beats if the heartbeats are corroborated by an associated non-electrophysiologicallysensed heart beat.

Heart rates, for example, may be computed based on both a succession ofelectrocardiogram signals and a succession of non-electrophysiologicsignals. These rates may be used to discriminate between normal sinusrhythm and the arrhythmia. The rates may be compared with arrhythmiathresholds, and used to determine absence of an arrhythmia, such as inresponse to a first rate exceeding a first arrhythmia threshold and asecond rate failing to exceed a second arrhythmia threshold. Thepresence of an arrhythmia may be determined using a morphology of theelectrocardiogram signals, and then verified using thenon-electrophysiologic signals.

In another embodiment of the present invention, defibrillation therapydelivery may be inhibited or withheld in response to detecting anarrhythmia using the electrocardiogram signals but failing to detect thearrhythmia using the non-electrophysiologic signal, such as a bloodsensor signal. A method of sensing an arrhythmia and inhibiting therapymay involve sensing an electrocardiogram signal at a subcutaneousnon-intrathoracic location. A detection window may be defined with astart time determined from the electrocardiogram signal. A signalassociated with a non-electrophysiological cardiac source may bereceived and evaluated within the detection window. The presence ornon-presence of a cardiac arrhythmia may be determined using theelectrocardiogram signal, and confirmed by the presence of the cardiacarrhythmia as detected by the non-electrophysiological cardiac signal.The start time of a detection window used for confirmation may beassociated with an inflection point of the electrocardiogram signal,such as a maxima or a minima. A correlation may be performed between theelectrocardiogram signal and the non-electrophysiological cardiacsignal.

Details of useful cardiac signal confirmation/verification techniquesinvolving non-electrophysiologic signals are disclosed in commonlyowned, co-pending U.S. Pat. Nos. 7,218,966 and 7,117,035, and USPublication No. 2005/0119708, which are hereby incorporated herein byreference.

According to one embodiment, photoplethysmography is used to aid innoise discrimination when detecting various heart rhythms in thepresence of electrical noise or artifacts. Because the additionaldiscriminating signal is based on blood oxygen level or pulsatile bloodvolume level, and not based on electrical cardiac signals, this signalmay provide information about a patient's rhythm state or hemodynamicseven in the presence of electrical noise.

A subcutaneous sensor may be used to detect blood oximetry. One suchsensor is a pulse oximetry sensor, for example. The blood oxygen levelinformation may be used together with rate, curvature, and other ECGinformation to discriminate normal sinus with electrical noise frompotentially lethal arrhythmias such as ventricular tachycardia andventricular fibrillation. An ITCS device may utilize the characteristicsof blood oxygen information combined with typical ECG information fordiscrimination.

In accordance with an embodiment of the present invention, subcutaneousphotoplethysmography may be employed to develop anon-electrophysiological cardiac signal for detection and/orconfirmation of cardiac rhythm. This feature employs a subcutaneousphotoplethysmogram as part of a subcutaneous ICD system (e.g., ITCSdevice) as an alternative or additional signal to the electrocardiogramfor detecting cardiac rhythm or hemodynamic state, particularly in thepresence of electrical noise.

Photoplethysmography may be used subcutaneously for confirming patientcardiac arrhythmia detected by an implantablecardiovertor/defibrillator. Subcutaneous photoplethysmography may alsobe used to characterize patient hemodynamics for an implantablecardiovertor/defibrillator. For example, subcutaneousphotoplethysmography may be used to evaluate afterload. Afterload is thesystolic load on the left ventricle after it has started to contract.The resistance associated with afterload results from resistive forcesof the vasculature that are overcome in order to push a bolus of bloodinto this vasculature during every heart beat. Hypertension or aorticstenosis could cause chronically increased afterload and lead to leftventricular hypertrophy and, subsequently, to heart failure.

Photoplethysmography may further be used subcutaneously for pulseoximetry to measure characteristics related to changes in patient oxygensaturation for an implantable cardiovertor/defibrillator. In general, itis desirable to reduce overall photoplethysmography energy, such as byutilizing it only for arrhythmia confirmation after other detectionalgorithms have been employed.

In one particular approach, a subcutaneous photoplethysmogram is used toconfirm or verify that the cardiac signal used for cardiac rhythmanalysis is indeed a cardiac signal, rather than a spurious signal, suchas a skeletal noise signal. For example, the subcutaneousphotoplethysmogram may be used to verify that the cardiac signal used tomake tachyarrythmia therapy delivery decisions is an electrocardiogramindicative of the patient's actual heart rhythm. According to thisapproach, the subcutaneous photoplethysmogram is used primarily forverifying that the signal used for arrhythmia analysis and therapydelivery decisions is indeed the cardiac signal, which is distinct fromusing this signal to separately verify the presence or absence of anarrhythmia. It is understood, however, that the subcutaneousphotoplethysmogram may be used as a signal to separately verify thepresence or absence of an arrhythmia, exclusively or in addition tousing this signal for cardiac signal confirmation.

For example, the control system processor may inhibit delivery of atachyarrhythmia therapy until the ECG signal used to detect presence ofthe arrhythmia is confirmed to include a cardiac signal (e.g., QRScomplex) using a photoplethysmic signal. The processor, for example, mayinhibit delivery of the tachyarrhythmia therapy for a predetermined timeperiod during which the verification processes is carried out, andwithhold delivery of the tachyarrhythmia therapy upon expiration of thepredetermined time period if such verification processes is unsuccessfulor in response to cessation of the arrhythmia. The processor may deliverthe tachyarrhythmia therapy in response to a successful outcome of theverification process. Also, the processor may immediately deliver thetachyarrhythmia therapy irrespective of the verification process inresponse to detection of a life-threatening arrhythmia.

Several benefits may be achieved through use of subcutaneousphotoplethysmography. For example, subcutaneous photoplethysmography maybe used to reduce the number of inappropriate shocks by improving shockspecificity. It may also be used to provide for confirmation ofventricular arrhythmias based on the level of blood perfusion orrelative change in blood perfusion. Further, subcutaneousphotoplethysmography may be used to complement cardiacelectrocardiogram-based algorithms by using a non-electric photo-baseddetection method. Subcutaneous photoplethysmography may also be used forredetection and reconfirmation of arrhythmias.

FIGS. 2 through 7 illustrate various embodiments and processesassociated with the use of subcutaneous blood sensing employed fordetection and/or confirmation of a cardiac rhythm. FIG. 2 illustratesone implementation of a photoplethysmographic sensing system 500suitable for use in a subcutaneous cardiac stimulator 510 (e.g., an ITCSdevice). FIG. 2 illustrates deployment of a subcutaneousphotoplethysmographic sensor 520 in an orientation between a layer ofskin 530 and a layer of muscle tissue 540. The illustrative example ofFIG. 2 depicts a light source 550 (i.e., LED) and a detector 560 facingtowards the muscle tissue 540. This orientation advantageously reducesinterference from ambient light sources, thus reducing noise artifactson the plethysmogram, particularly if an opaque barrier 570 is used todirect light into the detector 560. Other configurations may have thelight source 550 and the detector 560 on the side or facing towards theskin.

When the cardiac stimulator 510 encounters an electrocardiogram that itcannot interpret, or to confirm detection of a hemodynamically unstablearrhythmia, the light source 550 is activated and the output of thephotodetector 560 is synchronously measured. Algorithms in the cardiacstimulator 510 are then invoked to determine the pulse rate from thephotoplethysmogram and inform therapy decisions. Measurements from thissignal may also be used to inform or adapt electrocardiogram noisediscrimination and/or arrhythmia detection algorithms.

Use of subcutaneous photoplethysmography in accordance with thisembodiment advantageously provides for detection of cardiac rhythm inthe presence of electrical noise or artifacts. The algorithm is robust,in that the photoplethysmogram is an optical signal and therefore notsusceptible to the same noise sources as the ECG.

An expanded view 580 illustrates a light path 590 from the light source550 to the detector 560. The perfusion of blood in the muscle tissue 540affects the character of the light as it is reflected from the tissue540 to the detector 560 along the path 590, providing blood informationsuch as blood oxygen saturation level, blood volume, pulse, and otherblood characteristics.

The implementation shown in FIG. 3 includes an optical source circuit515 that includes an LED control 525 respectively coupled to a Red LED535 and an Infrared (IR) LED 545. Changes in oxygen saturation levels inthe tissue 542 may be measured using two light sources and one detector.Typically, one light source has absorption characteristics that aregenerally unaffected by blood color change (such as the IR LED 545,emitting at ˜960 nm), while the other light source has absorptioncharacteristics that are sensitive to color change in the blood (such asthe Red LED 535, emitting at ˜660 nm). Due to potential errors incalculating absolute oxygen saturation using reflectance in areas of lowperfusion, the embodiments illustrated in FIGS. 3 through 6 only monitorchanges in oxygen saturation, and not absolute levels. The informationfrom changes in oxygen saturation of the blood is sufficient todiscriminate between potentially lethal arrhythmias and noise artifactsthat could otherwise lead to unnecessary shocking of the patient withoutthe discrimination.

Still referring to FIG. 3, an optical detection circuit 555 includes adetector 565 coupled to a photo diode 576. Processing circuitry 575 iscoupled to the optical source circuit 515 and the optical detectioncircuit 555 in this configuration. The processing circuitry 575 includesa multiplexer 585 coupled to the LED control 525 and the detectioncircuit 555. A Red signal channel 586 and IR signal channel 587 arerespectively coupled between the multiplexer 585 and the signalprocessing circuitry 575. The signal processing circuitry 575 operateson signals received from the Red signal channel 586 and the IR signalchannel 587, and employs various algorithms to evaluate such signals forcardiac rhythm detection and/or confirmation, including arrhythmiadetection and confirmation.

A magnified view 582 illustrates a light path 572 from a first lightsource 552 and a light path 574 from a second light source 554 to adetector 562. The perfusion of blood in the muscle tissue 542 affectsthe character of the light as it is reflected from the tissue 542 to thedetector 562 along paths 572 and 574, providing blood information suchas blood oxygen saturation level, blood volume, pulse, or other bloodcharacteristics.

FIGS. 4 and 5 are graphs of data taken from a live porcine subject, andillustrate an example of combining electrocardiography andphotoplethysmography to differentiate normal sinus rhythm fromarrhythmia in accordance with an embodiment of the present invention.FIG. 4 illustrates a cardiac electrocardiogram 700 and aphotoplethysmogram 710 presented over a 2 second duration for a normalsinus rhythm condition 730 and a ventricular fibrillation condition 740.FIG. 5 illustrates a cardiac electrocardiogram 760 and a time correlatedphotoplethysmogram 770 during a 38 second period 780 in which a normalsinus rhythm 762 is followed by a ventricular fibrillation event 764.FIGS. 4 and 5 demonstrate that both the cardiac electrocardiograms 700,760 and photoplethysmograms 710, 770 change significantly in characterwhen the normal sinus rhythm 730, 762 devolves into the ventricularfibrillation 740, 764 conditions.

Referring back to FIG. 4, note that the scale of the normal sinus rhythm730 graph and the ventricular fibrillation 740 graph are different.Although the photoplethysmogram 710 of the ventricular fibrillation 740looks comparable to the photoplethysmogram 710 of the normal sinusrhythm 730, the peak-to-peak amplitude of the photoplethysmogram 710 inthe ventricular fibrillation 740 graph is significantly smaller than thepeak-to-peak amplitude of the photoplethysmogram 710 in the normal sinusrhythm 730 graph. The ordinate scale of the ventricular fibrillation 740graph is equal to the ordinate scale of the normal sinus rhythm 730graph.

Referring now to FIG. 5, a RMS blood oxygen level 772 corresponds to thenormal sinus rhythm 762, and a RMS blood oxygen level 774 corresponds tothe ventricular fibrillation event 764. A threshold 776 may bepredetermined or adaptively adjusted to help differentiate between thenormal sinus rhythm 762 and the ventricular fibrillation event 764. Thetime period between the normal sinus rhythm 762 and the ventricularfibrillation event 764 indicates a loss of data in the electrocardiogram760 during the intentional induction of the ventricular fibrillation764.

FIG. 6A is a schematic of an LED current source section 810 of aphotoplethysmography circuit in accordance with an embodiment of thepresent invention. As is illustrated in FIG. 6A, the current sourcesection 810 is configured as a constant current source, using a sourceLED circuit 811, and is driven by an oscillator 812 that may producedrive pulses 813 having a period of 1 ms and a pulse width of 0.1 ms,for example.

FIG. 6B is a schematic of a photo detector section 820 of aphotoplethysmography circuit in accordance with an embodiment of thepresent invention. The detector section shown in FIG. 6B includes aphoto diode 821, a light current to voltage amplifier 822, a high passfilter 823, a voltage integrator 824, and a low pass filter 825. Thecircuits illustrated in FIGS. 6A and 6B are useful for providing aphotoplethysmic signal, such as signal 770 shown in FIG. 5.

FIG. 7 illustrates various processes associated with one method ofutilizing subcutaneous photoplethysmography in combination withelectrocardiogram-based rhythm detection. The method illustrated in FIG.7 presents details concerning energy utilization. Thephotoplethysmography circuitry may be enabled only after otherarrhythmia detection methods have been employed, such as cardiacelectrocardiogram-based algorithms. Photoplethysmography may be usedonly before potentially delivering the shock, to conserve energy. Thecircuitry may be disabled when use of the photoplethysmogram iscompleted. According to one implementation, if photoplethysmography isused for 10 seconds, the additional energy required is about 0.5 joules.This energy is very low when compared with the energy used fordefibrillation (>5 joules). Therefore, discriminating oneelectrocardiogram identified arrhythmia event as noise usingphotoplethysmography has the potential to save over 4.5 joules. It isunderstood that eliminating unnecessary shocks extends the useful lifeof the ITCS, while simultaneously improving patient comfort.

With reference to FIG. 7, and with further reference to FIGS. 4 and 5,an ECG-based detection algorithm 600 is employed to detect cardiacarrhythmias. If a ventricular arrhythmia is detected 602 using ECG baseddetection 601, a determination 604 is performed to see if thephotoplethysmogram has been checked. A check 606 of an acquiredphotoplethysmogram is performed. If the photoplethysmogram indicates orconfirms the presence of a ventricular arrhythmia, such as by using athreshold 607, the defibrillation capacitor is charged 608 and a shockis delivered 610. It is noted that a ventricular arrhythmiare-verification routine may be performed during capacitor charging priorto shock delivery.

If the photoplethysmogram signal exceeds the predetermined threshold607, such as the threshold shown in FIG. 5 (note that an RMS level ofthe photoplethysmogram may be used in this comparison), a recheck 614 ofthe ECG signal is made after a predetermined time period.

In the methodology depicted in FIG. 7, the photoplethysmic sensor thatproduces the photoplethysmogram signal may be selectively powered-up andpowered-down. For example, the photoplethysmic sensor may be in apowered-down state until a tachyarrhythmia is detected using the ECGsignal, such as at blocks 601 and 602 in FIG. 7. The photoplethysmicsensor may remain powered-on until completion of the cardiac signaland/or arrhythmia detection verification processes. For example, thephotoplethysmic sensor may be powered-down after completing theprocesses associated with blocks 606 and 607, and prior to charging thedefibrillation capacitor at block 608, which may take as long as about20 seconds to fully charge the capacitor(s).

Various modifications and additions can be made to the preferredembodiments discussed hereinabove without departing from the scope ofthe present invention. Accordingly, the scope of the present inventionshould not be limited by the particular embodiments described above, butshould be defined only by the claims set forth below and equivalentsthereof.

1. (canceled)
 2. An implantable subcutaneous defibrillator comprising: ahousing configured for subcutaneous placement; detection circuitryprovided in the housing and configured to produce an electrocardiogramsignal; energy delivery circuitry provided in the housing and configuredto provide a defibrillation therapy output; at least one electrodeconfigured for subcutaneous non-intrathoracic placement and coupled tothe detection and energy delivery circuitry; wherein the implantablesubcutaneous defibrillator is configured to communicate with a secondimplantable device; wherein the implantable subcutaneous defibrillatoris configured to operate in the following manner: using the detectioncircuitry, determine ether a potentially treatable condition isoccurring; using the communications circuitry, request information fromthe second implantable device relating to the potentially treatablecondition; and after requesting information from the second implantabledevice, either: confirm the potentially treatable condition and delivertherapy using the energy delivery circuitry; or using informationreceived from the second implantable device, determine that thepotentially treatable condition is not to be treated.
 3. The implantablesubcutaneous defibrillator of claim 2 further comprising communicationscircuitry for establishing a short range wireless communication link. 4.The implantable subcutaneous defibrillator of claim 2 wherein the secondimplantable device is a non-electrophysiologic sensor, and theimplantable subcutaneous defibrillator is configured to request datarelated to sensed non-electrophysiologic conditions from the secondimplantable device.
 5. The implantable subcutaneous defibrillator ofclaim 4 wherein the second implantable device is configured for pulseoximetry, and the implantable subcutaneous defibrillator is configuredto compare electrocardiogram signals from the detection circuitry topulse oximetry data from the second implantable device.
 6. Theimplantable subcutaneous defibrillator of claim 4 wherein the secondimplantable device is configured for sensing an acoustic signal, and theimplantable subcutaneous defibrillator is configured to compareelectrocardiogram signals from the detection circuitry to acousticsignal data from the second implantable device.
 7. The implantablesubcutaneous defibrillator of claim 2 second implantable device is anelectrophysiologic sensor, and the implantable subcutaneousdefibrillator is configured to request data related to sensedelectrophysiologic conditions from the second implantable device.
 8. Amethod of making a therapy decision in an implantable subcutaneousdefibrillator having a housing for subcutaneous placement with at leastone electrode coupled thereto and detection and therapy deliverycircuitry therein, the method comprising: sensing an electrocardiogramvia the at least one electrode and using the detection circuitry;determining a cardiac arrhythmia is present by analyzing theelectrocardiogram; obtaining information from a second implantabledevice; confirming the presence of the cardiac arrhythmia using theinformation from the second implantable device; and delivering therapyin response to the confirmed cardiac arrhythmia.
 9. The method of claim8 wherein the second implantable device is a non-electrophysiologicsignal detector, and the step of confirming the presence of the cardiacarrhythmia includes setting a detection window and evaluating anon-electrophysiologic signal from the second implantable device withinthe detection window.
 10. The method of claim 9 wherein the secondimplantable device is configured to sense blood oxygen levels.
 11. Themethod of claim 9 wherein the second implantable device is configured tosense pulsatile blood volume.
 12. The method of claim 8 wherein thesecond implantable device is a non-electrophysiologic signal detector,and the step of confirming the presence of the cardiac arrhythmiaincludes analyzing a correlation between the electrocardiogram signaland the non-electrophysiologic signal data.
 13. The method of claim 12wherein the second implantable device is configured to sense bloodoxygen levels.
 14. The method of claim 12 wherein the second implantabledevice is configured to sense pulsatile blood volume.
 15. A method ofmaking a therapy decision in an implantable subcutaneous defibrillatorhaving a housing for subcutaneous placement with at least one electrodecoupled thereto and detection and therapy delivery circuitry therein,the method comprising: sensing an electrocardiogram via the at least oneelectrode and using the detection circuitry; determining a cardiacarrhythmia is present by analyzing the electrocardiogram; obtaininginformation from a second implantable device; rejecting the presence ofthe cardiac arrhythmia using the information from the second implantabledevice; and inhibiting therapy in response to the confirmed cardiacarrhythmia.
 16. The method of claim 15 wherein the second implantabledevice is a non-electrophysiologic signal detector, and the step ofrejecting the presence of the cardiac arrhythmia includes setting adetection window and evaluating a non-electrophysiologic signal from thesecond implantable device within the detection window.
 17. The method ofclaim 16 wherein the second implantable device is configured to senseblood oxygen levels.
 18. The method of claim 16 wherein the secondimplantable device is configured to sense pulsatile blood volume. 19.The method of claim 15 wherein the second implantable device is anon-electrophysiologic signal detector, and the step of confirming thepresence of the cardiac arrhythmia includes analyzing a correlationbetween the electrocardiogram signal and the non-electrophysiologicsignal data.
 20. The method of claim 19 wherein the second implantabledevice is configured to sense blood oxygen levels.
 21. The method ofclaim 19 wherein the second implantable device is configured to sensepulsatile blood volume.